At the end of each step of drilling a well, whether in an onshore or offshore field, a casing string must be placed in the borehole and pass through successive geological layers, comprised of the various types of rock formed over thousands of years and different environmental conditions and geological processes. Thus, they may have different mineralogical compositions, geomechanical properties and field orientations/stresses.
A borehole will receive successive columns during the course of its lifetime, placed during the different steps of the well drilling process, until the crude oil reservoir to be explored is reached. These retain a cylindrical cross-section for a certain period of time. However, as the borehole penetrates successive lithological layers of differing geomechanical properties and different stresses, the walls of the column within each section may be deformed, losing their original circular cross-section, or the column axis may be displaced relative to the columns in the sections immediately above or below it.
It is common knowledge that the oil industry has operated in ultra-deep salt layers. A marked characteristic of this mineral is its remarkable ability to creep and slowly deform under steady-state variables over a short period of time, even under low stress and temperature conditions.
From the point of view of exploration, the presence of salt rocks in hydrocarbon prospects increases the likelihood of success, as these sediments may deform and create diverse structures that favor the accumulation of hydrocarbons. Furthermore, they are practically impermeable and thus make good cappers.
However, when it comes to drilling oil wells, compared to other lithologies, the presence of salt rock is associated with additional problems due to creep.
This physical phenomenon is measurably influenced by deviatoric stresses, the absolute temperature and the type of salt. The greater the depth and thickness of the layer of salt rock that must be drilled through, the greater will be the deviatoric stress and the temperature, and thus it is more likely that the borehole will close over time.
Typical consequences one can anticipate range from restricting the passage of the drilling column, to irreversible entrapment requiring a change in the course of the borehole or even abandoning the well altogether. However, over the medium and long term, salt creep also places additional loads on the casing strings, and may cause ovalisation, thus limiting the passage of the tools required to for well completion and production.
Depending on the degree of ovalisation of the casing string, it can become impossible to remove the production equipment from within the well (installed within the casing string), or it can become damaged, resulting diminished or even no production from the well. Depending on the situation of a given borehole, salt creep can stress the casing string in unexpected directions, and can even result in rupture or collapse of the string if it was not properly sized.
Thus the lifetime of the well, which is designed to last for 20 to 30 years, is drastically reduced under such a severe scenario, subject to unexpected stresses. Oftentimes this requires building a new, unplanned borehole that was not included in the budget to develop the oilfield.
Thus salt creep must be taken into consideration over the entire lifetime of the well as a function of the casing used and the conditions for building and operating the well; one must especially take into consideration a number of events and conditions that may occur during the lifetime of the well, changing the stresses and the temperature to which it is subject.
For example, the start of production increases the temperature along the length of the well, while maintenance operations may result in decreased pressure inside the pipe, both of which may be detrimental to pipe integrity, given the increase in the creep rate of the evaporite rock surrounding the borehole, and a reduction of the internal reaction force the pipe exerts on the salt layer.
Injecting hot steam into boreholes, a process that is routinely used to recover crude oil, and the resulting corrosion of the coating wall, are further unfavorable events, similar to those mentioned in the previous paragraph, and that also play an important role in the structural integrity of pipe.
Numerous authors have studied the resistance to failure of casing strings subject to different non-uniform load conditions, compared to their resistance when submitted to hydrostatic loads according to specifications published by the American Petroleum Institute (API).
These studies were motivated by the failure of casing strings in salt layers, detected a few weeks to a few years after they had been installed in oil wells. It is assumed that failures are the result of non-uniform loads placed on the casing due to incomplete cementing of borehole with enlarged boreholes (extra borehole caliper), as seen in consecutive borehole profiles.
On the other hand, it has been found almost empirically that in some cases, in order to offer the same resistance to collapse, casings submitted to non-uniform loads must have wall thicknesses 3 or 4 times the wall thickness of casings submitted to hydrostatic loads.
As the creep of evaporite rocks is substantially influenced by temperature and stress, the deeper the layer of the evaporitic rock to be traversed by the borehole, the greater will be the deviatoric stresses and temperature acting on the well.
However, because of the different chemical composition and microstructures that characterize the diverse types of evaporite created by nature, they are expected to present different creep behaviors when submitted to the same boundary conditions. For this reason, the load placed on casing strings due to salt creep can vary widely with borehole depth.
In general, boreholes containing regions that are whorled or with extra borehole caliper (widened), whether or not these are associated with off-center borehole casings, may not enable suitable cleaning of the borehole wall liner, resulting in gaps in the cementing paste (channeling), resulting in poor cementing of this region and the one above it. Consequently, the coating may be subjected to non-uniform and spot loads; these conditions are not among the criteria used to calculate the collapse performance of coatings in API 5CT (ISO 11960), which is based on uniformly distributed radial loads.
Resistance to collapse in pipes is a complex combination of a number of variables: geometric characteristics, the properties of the metal the pipe is built of, the loads applied and the means used to secure the pipe.
Among the geometric factors, we have the internal pipe diameter, wall thickness, ovalisation, eccentricity and variations in pipe thickness.
Among the mechanical properties of the material, we have yield stress, elasticity module, the shape of the stress-strain curve and residual stresses.
The loads applied may be associated with an external load acting circumferentially to other loads such as axial compression and traction, bending, torsion and internal pressures.
Regarding how pipes are fixed, we have the length of free (non-secured) pipe that is subject to the forces, and the contact area with rock.
However, testing equipment currently available to determine the collapse load of pipe enables assembling only a section of pipe in a chamber, which is then submitted to increasing hydrostatic loads for a period of time until collapsing.
Collapse pressure is calculated using analytical equations specified in API 5CT (ISO 11960), which are based on uniformly distributed radial loads.
Thus, currently available hydrostatic chambers present shortcomings, especially as they put only hydrostatic pressure on the pipe, which is not representative of the phenomena that may actually take place in salt zones and in regions with stress anisotropy.
Although it is possible, using currently available digital technology and computational modeling, to create mathematical models that simulate the various non-uniform forces, and use these virtual models to define sufficient data parameters to design casing strings under different boundary conditions, this data cannot be proven using the currently available hydrostatic chambers.
Documents US 2008/0034885 A1 and U.S. Pat. No. 7,051,600 show some examples of equipment capable of submitting a structure to multiple forces, however this equipment is not sufficient to generate simultaneous and non-uniform forces along a structure, and are thus unable to validate the values obtained from using numerical modeling.
Document U.S. Pat. No. 7,669,482 describes equipment capable of applying loads, displacements, temperature and pressure on pipes to simulate the conditions at the bottom of a well. However, in addition to being a large and expensive piece of equipment, the description contains none of the internal details of the apparatus.
Document BR 020100121966 of 30 Dec. 2010, by the same depositor, shows equipment with components suitably built to submit a structure to multiple, non-uniform efforts simultaneously, and thus suitable for validating the values obtained using the numerical modeling in question with a large degree of precision. However, this too is a large and expensive device built to deliver great performance and focused on testing any longilinear structure, not only to simulate the results of cementing failures, but also for numerous other simulations such as creep, buckling and torsion, among others.
Thus, although there exists suitable technology for submitting a structure to non-uniform loads that can validate the values obtained from numerical models, the continuous search for savings and to use the resources available at industrial facilities has led to research into new techniques.
In light of this technical challenge, there emerged a concern with developing a method capable of using technological resources already available, so as to deploy them to validate the results obtained from computer models that are as close as possible of the actual situation found in structures designed for oil wells.
Thus, research has focused especially on using the largest and most conventional test equipment available right now for oil well casing strings: hydrostatic chambers.
It is known that the currently available hydrostatic test chambers are capable of submitting oil well casing strings to homogeneous loads only, and to date there have been no techniques for using this load analysis device in such a way as to provide the means to simulate loads applied in a non-uniform manner along the length of oil well casing strings. The object of the invention described herein is to structure a method to apply non-uniform loads to structures using a conventional hydrostatic chamber.
Other objectives the present invention proposes to reach are:                a) enable the application of non-uniform loads to a structure;        b) reduce casing string failure rate;        c) enable conducting tests that are as close as possible to the actual situation under which the structures designed for oil wells will operate;        d) ensure certification of the structural integrity of the borehole built.        